The objective of the proposed research is to develop a sufficient understanding of the snail-trematode interactions to be able to utilize this approach as a means for developing biological control methods. Emphasis will be given to the study of acquired and natural resistance to trematode infections in snails. With the use of appropriate snail-trematode systems, appropriate combinations of irradiated and nonirradiated trematode species, controlled dosage and infection intervals, along with standard histological and electron microscopic techniques, we shall study the underlying principles of acquired and natural resistance. This includes the study of the amebocyte-producing organ; the study of interactions between the snail's resistance and the parasite's interference capacity; the study of the function of the hyalinocytes, granulocytes and the humoral factors as part of the internal defenses of the snail.